This starting pitching depth that they have gives the A's the opportunity to be patient with Harden. The only thing about the A's current rotation is the fact that they have 4 left handers and that so far has not proven to be a bad thing. I would assume that at the first need for another starter that Harang would get the call, but I don't think that they could go wrong either way with Harang or Harden.
A side note, if Halama keeps letting baserunners on at his current rate, he won't be keeping a sub-4 ERA for long. But that's the biggest current issue that I can think of at the time with their current 5.